Influenza Pandemics More Likely to Occur Outside Flu Season

Influenza pandemics can emerge unexpectedly and wreak global devastation. However, each of the six pandemics since 1889 emerged in the Northern Hemisphere following the flu season, suggesting that pandemic timing may be predictable. Fox and colleagues provide a possible explanation that seasonal flu epidemics may leave a wake of transient immunity in the population hindering pandemic emergence. Hypothetical seasonal flu epidemic spread (not based on real or simulated data) is depicted here with the colors indicating regions currently infected with seasonal flu (red), refractory and immune to pandemics (purple), and recovered and currently susceptible to a novel pandemic (blue). White lines depict the global flight network. (Spencer J. Fox)

(CN) – Influenza pandemics in the Northern Hemisphere are most likely to emerge outside of the traditional flu season, according to a new study.

One might assume that the greatest risk of an influenza pandemic would be during the Northern Hemisphere’s normal flu season between October and May, when influenza viruses are most abundant and likely to circulate. However, all six flu pandemics that have spread north of the equator since 1889 emerged during spring and summer months.

The seasonal timing of those pandemics could stem from two opposing factors: the spread of the flu under winter’s ideal environmental and social conditions, and the temporary immune protection humans develop in response to infections from other flu viruses, a team of scientists reports Thursday in the journal PLOS Computational Biology.

The researchers tested this hypothesis by creating a computational model that mimics viral spread during flu season, which also factors in the potential for infected individuals to develop long-term immunity to seasonal flu and temporary immunity to emerging pandemic viruses.

Using real-world data on flu transmission from the 2008-09 flu season, the model correctly predicted the timing of the 2009 H1N1 pandemic.

The team used their model to run thousands of simulations in which new pandemic viruses occurred at various points during flu season. The results of these simulations supported their hypothesis, as the combination of cross-virus immunity and supportive winter conditions led to spring and summer pandemics, according to the study.

“We don’t know when or where the next deadly flu pandemic will arise,” said lead investigator Lauren Ancel Meyers, an integrative biologist at the University of Texas at Austin. “However, the typical flu season leaves a wake of immunity that prevents new viruses from spreading.

“Our study shows that this creates a narrow, predictable window for pandemic emergence in the spring and early summer, which can help public health agencies to detect and respond to new viral threats.”

Future studies may focus on characterizing the cross-strain immunity that hinders pandemic emergence during the traditional flu season, according to the team. Researchers may also examine how pandemic risk patterns occur in both tropical regionals and the Southern Hemisphere.


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